The present invention concerns a new process for the production of aminoplastic resins having very low formaldehyde emission rates. Its subject is more particularly a process for the production of urea-formaldehyde melamine "doped" resins which give off very little formaldehyde.
Urea-formaldehyde resins are known products which are widely used in the wood industry in particular for the production of particle boards. They are produced in a known manner by the condensation of urea and formaldehyde at pH between 4 and 7 and at a temperature near boiling point: this condensation reaction is preferably carried out in several stages.
The main drawback associated with urea-formaldehyde resins is that they give off large amounts of free formaldehyde. Attempts have been made to reduce the free formaldehyde content using various production processes: unfortunately, when particularly low formaldehyde contents are aimed at, experience shows that this objective is accompanied both by a decrease in reactivity and stability of the resins, and by deterioration of the mechanical properties of the finished boards. In order to avoid the presence of free formaldehyde, it has also been suggested that formaldehyde-free resins should be used, in particular resins based on isocyanate solutions. Unfortunately isocyanates are liable to pose toxicological problems.
Another way of limiting formaldehyde emissions consists in adding substances that fix this compound. Such substances would for instance include urea, melamine or phenol, or else high-protein flours. Unfortunately all processes that involve adding these formaldehyde-fixing additives to urea-formaldehyde resins have troublesome secondary effects, in particular, they retard the final condensation and hence the hardening of the glues.
There is therefore a need for the development of urea-formaldehyde resins having good stability and lower rates of emission of free formaldehyde, and whose utilization yields finished particle boards with good mechanical characteristics.